Support for project-local migration templates

When running migrations on production databases with active traffic, it's important to include safety settings like `SET lock_timeout = '5s';` to prevent migrations from blocking writes indefinitely while waiting for locks.

Currently, teams must remember to manually add this to every migration - which is error-prone. A simple template feature would allow enforcing safe defaults automatically.

**Proposed solution:**

If a template file exists in the project (e.g., `db/.dbmate/template.sql` or configured via `--template-file`), `dbmate new` would use it instead of the default empty template:

```sql
-- migrate:up
SET lock_timeout = '5s';

-- migrate:down
```

**Why this differs from PR #530:**

The closed PR stored templates in `~/.config`, which goes against dbmate's KISS philosophy. This proposal keeps templates **in the project repo**, making them:
- Version controlled with the project
- Shared across the team automatically
- Consistent with dbmate's simple, portable approach

This is a small change that would help teams enforce safe migration practices without external tooling.